Conventionally, an optical disk such as a CD, DVD or BD (Blu-ray disc) is used widely for recording and reproducing video image signals and sound signals. In an optical disk apparatus which records or reproduces information to and from an optical disk, processing for reading out information that has been written to an information layer of an optical disk is carried out by scanning very fine tracks by means of a very small optical beam spot which has been made to converge on the information layer by an optical pickup. In this case, in reading out the information written on an optical disk accurately and continuously, servo technology for causing the optical beam spot to follow a track is indispensable.
Consequently, in an optical disk apparatus, focus control is generally carried out to cause an optical beam spot to follow an information layer on an optical disk on the basis of a focus error signal (hereinafter called “FE signal”) which represents positional deviation between the optical beam spot and the information layer. In addition, tracking control is carried out to cause the optical beam spot to follow a track on the basis of a tracking error signal (hereinafter called “TE signal”) which represents positional deviation between the optical beam spot and the center of the track.
Furthermore, even if the reflectivity of the optical disk varies or the light beam irradiation power varies during recording or reproduction, generally, the determination gain of the FE signal and the TE signal are kept uniform by an AGC (Automatic Gain Control) circuit which normalizes the FE signal and the TE signal by the quantity of the returned light. As a result of this, it is possible to achieve stabilized focus control and tracking control.
Therefore, when recording or reproducing information to or from an optical disk having a cover layer and a plurality of information layers, in an optical disk apparatus of this kind, the following issues arise.
For example, in an optical disk having a cover layer and a plurality of information layers, when a light beam is irradiated onto a prescribed information layer in order to reproduce information, the returned light of the determined light beam includes reflected light from the surface of the optical disk and other information layers (other layer stray light), as well as reflected light from the prescribed information layer. Consequently, the amount of returned light determined is the sum of the reflected light from the prescribed information layer and other layer stray light, and the amount of returned light cannot be determined accurately. Here, if the optical disk has a cover layer and one information layer, then the other layer stray light is reflected light from the surface of the cover layer (the surface of the optical disk), and the amount of reflected light from the surface of the cover layer depends on the surface reflectivity and the thickness of the cover layer. In other words, the reflected light becomes greater, the higher the surface reflectivity, and the reflected light becomes greater, the smaller the thickness of the cover layer.
As a result of this, normalization for correcting variation in the reflectivity in the prescribed information layer of the optical disk and normalization for correcting variation in the light beam irradiation power during recording or reproduction are not carried out correctly in the AGC circuit, and an FE signal and a TE signal having a suitable determination gain are not obtained. Consequently, the reflected light from the surface of the cover layer leads to destabilization of the focus control and tracking control, and gives rise to decline in the recording characteristics and reproduction characteristics of the optical disk apparatus.
In order to solve problems of this kind, an optical pick-up has been proposed which corrects the amount of returned light and generates an error signal having reduced effects of other layer stray light, by providing a dedicated detector for determining the other layer stray light, subtracting the determined amount of light for the other layer stray light from the determined amount of light for the returned light and using the resulting amount in the AGC circuit (see, for example, Patent Literature 1).
On the other hand, in recent years, methods and apparatuses which achieve further increase in capacity of optical disks have been developed. In an optical disk apparatus, the information recording density depends on the size of the light beam spot which converges on the recording medium. Therefore, increased capacity of an optical disk can be achieved by reducing the size of the light spot which is irradiated by the optical pick-up.
The size of the light spot is proportional to the numerical aperture of the object lens and inversely proportional to the wavelength of the irradiated light. Therefore, in order to obtain a smaller light spot, either the wavelength of the light used should be made shorter or the numerical aperture of the object lens should be made greater. However, in optical information recording and reproduction apparatuses which have been developed for practical use thus far, the interval between the optical disk and the object lens differs from the light wavelength by a sufficiently large amount, and when the numerical aperture of the object lens exceeds 1, the light emitted from the object lens is fully reflected by the emission surface of the lens and therefore it has not been possible to raise the recording density.
Therefore, as an optical recording and reproduction method using an object lens having a numerical aperture exceeding 1, a near-field optical recording and reproduction method using a SIL (Solid Immersion Lens) has been developed. The numerical aperture NA is expressed by NA=n·sin θ, taking the refractive index of the medium to be n and taking the maximum angle with respect to the optical axis of the light beam in the medium to be θ. Normally, when the numerical aperture is greater than 1, the angle θ becomes equal to or greater than the critical angle, and therefore the light in this region is fully reflected at the emission end face of the object lens. This fully reflected light spills out from the emission end surface as evanescent light. In a near-field optical recording and reproduction method, it is possible to transmit this evanescent light to the optical disk from the lens. Therefore, the interval between the emission end face of the object lens and the surface of the optical disk (the air gap) is kept at a distance which is no more than ¼ of the wavelength of the optical beam, in other words, a distance which is shorter than the attenuation distance of the evanescent light, and hence light in a range where the numerical aperture exceeds 1 is transmitted through the optical disk from the object lens.
In an optical disk apparatus which uses a SIL in this way, conventionally, a composition is adopted in which information is recorded or reproduced to and from an optical disk having an information layer provided on the surface thereof. However, from the viewpoint of protecting the information layer, a desirable composition is one which records or reproduces information to and from an optical disk provided with a cover layer, as in existing optical disks.
If information is recorded to or reproduced from an optical disk having a cover layer in an optical disk apparatus using a SIL, the following issues arise.
FIG. 17 is a diagram showing a relationship between a near-field air gap and the level of surface reflected light, in a conventional optical disk apparatus. In FIG. 17, the horizontal axis Gap is the air gap and the vertical axis Ref is the reflectivity. Here, the reflectivity indicates the ratio of surface reflected light with respect to the amount of light in the light beam which is incident on the object lens.
As shown in FIG. 17, when the wavelength of the optical beam is 405 nm, then in the region where the air gap is approximately 100 nm or less, the surface reflectivity varies dramatically in accordance with the air gap and the surface reflectivity reaches a maximum of approximately 37%. On the other hand, in an existing optical disk apparatus, the surface reflectivity is around 8% and is uniform, regardless of the air gap.
Moreover, in an optical disk apparatus using a SIL, it is necessary for the cover layer of the optical disk to have a thickness of several μm approximately. On the other hand, in an existing optical disk apparatus, the cover layer of a BD which has the thinnest cover layer has a thickness of 100 μm.
From the foregoing, in the case of an optical disk apparatus using a SIL, the effects of the surface stray light (surface reflected light) are large compared to an existing optical disk apparatus, in other words, an optical disk apparatus using a far-field optical recording and reproduction method. For instance, in the case of an optical disk apparatus in which the numerical aperture is 1.78, the refractive index n of the SIL and the cover layer is 2, and the thickness of the cover layer is approximately 1.2 μm, the ratio between the reflected light from the information layer and the stray light from the surface of the cover layer reaches a value of around 2:1. Therefore, the effects of the stray light from the surface of the cover layer are extremely large.
However, the prior art does not disclose a method for determining surface stray light in an optical disk apparatus using a SIL.